Technical Field
The present invention relates to a hybrid shovel that assists an engine by means of an electric motor and a control method thereof.
Description of Related Art
In a shovel, normally, an engine is controlled to maintain a constant revolution speed. When a load of a hydraulic pump is applied to the engine, a fuel injection is performed to increase a torque of the engine so as to perform a control for maintaining the revolution speed of the engine. However, if the load of the hydraulic pump is sharply increased, an increase in the engine torque cannot follow the load increase, and there may be a case where the engine revolution speed is temporarily decreased. In this case, a fuel injection is needed to return the engine revolution speed to the original revolution speed. As a result, not only the fuel consumption is deteriorated but also the engine revolution speed is decreased, and, thereby, the movement of a drive cylinder becomes worse, which results in an occurrence of slowness.
There is suggested a shovel in which an engine is assisted by controlling an assist motor to suppress an occurrence of such a problem. Such a shovel is generally provided with an electric motor (assist motor) that outputs a power for driving a hydraulic pump to assist an engine. Then, it is suggested to return the engine revolution speed to the constant revolution speed by decreasing the engine revolution speed by driving the assist motor to assist the engine even when a load of the hydraulic pump sharply increases.
However, although the engine revolution speed is returned by assisting the engine by the assist motor, if the entire torque to return the engine revolution speed is provided by the assist motor, the engine itself may lose a power to return to the original revolution speed. Thus, the engine torque may always be a torque from which the torque of the assist motor is subtracted, and it may fall into a condition that, if the assist by the assist motor is stopped, the revolution speed is reduced again.
That is, if the engine is assisted by means of the assist motor even though the engine itself can increase the revolution speed, a control to attempt to return the revolution speed by increasing the fuel injection amount of the engine may not be performed.
Thus, it is desirous to develop a technique that can return the revolution speed to the original constant revolution speed by its own effort even when an assist is performed by the assist motor because of a reduction in the engine revolution speed.